The present invention generally relates to cooling systems for electronic components. More particularly, this invention relates to a sealed cooling device that contains a fluid and a mesh material through which the fluid flows to promote heat transfer through the device.
Cooling of electronic devices has become increasingly challenging as electronics have evolved. As manufacturing processes are constantly refined, the migration to smaller design processes and the incumbent reduction in operating voltage has not kept pace with the increased complexity of faster integrated circuits (ICs). Increasing number of transistors in combination with increasing operating frequencies has resulted in higher numbers of switching events over time per device. As a result, within the same market space and price range, ICs are becoming more and more sophisticated and power-hungry with every generation.
Compared to earlier generations, the implementation of smaller design processes has allowed the integration of more electronic building blocks such as transistors and capacitors on the same footprint. Consequently, area power densities have increased, resulting in smaller dies dissipating higher thermal load. As a result, formerly sufficient, passive heat spreaders and coolers often do not provide adequate cooling. While sophisticated fin designs and powerful fans increase the active surface area useable for offloading thermal energy to the environment, even extremely well designed coolers are hitting inherent limitations. In particular, significant limitations stem from the bottleneck of limited heat conductivity of the materials used, and specifically the fact that passive heat transfer throughout a solid structure is limited by the thermal conductance coefficient of the material and the cross sectional area of the structure.
In a two-dimensional heat spreader of uniform thickness, the amount of heat energy decreases as a square function of the distance from the source, where the thermal conductance coefficient of the material and the cross sectional area define the slope of the decrease. Therefore, even the most highly conductive material will not be able to maintain an even temperature distribution across the entire surface of the cooling device. Any gradient, on the other hand, will cause a decrease in cooling efficiency since the temperature difference (ΔT) between the cooler's surface and the environment is the primary limiting factor for thermal dissipation to the surrounding.
In view of the above, it is desired that coolers transfer heat from a heat source as quickly and efficiently as possible to provide a uniform temperature distribution or isothermicity at the cooler's surface. In combustion engines, liquid cooling has become the method of choice, using the fact that a liquid (e.g., water) is taking up thermal energy and subsequently being pumped to a remote radiator where it releases the absorbed heat. In electronic devices, liquid cooling is still only marginally accepted for reasons that include the inherent risk of spills, cost overhead, and complexity of the installation, which involves routing of tubing and installation of radiators. Alternatively, some self-contained liquid cooling devices have been proposed and marketed.
The four primary factors defining the efficacy of a liquid cooling device are the uptake of heat by the cooling fluid at the heat source, the transport rate of the fluid away from the heat source, the offloading of heat to the solid components of the cooler, and finally the dissipation rate of heat into the environment. The exchange of heat between the fluid and the cooling device largely depends on the routing of the flow of the coolant within the device. If the channels are too wide, laminar flow can cause a decrease in efficacy of heat exchange between the fluid and the device. Therefore, it is desirable to have a capillary system to achieve an optimal surface to volume ratio. Such capillary systems have been referred to as microchannel systems.
Different technologies have been employed to create microchannels, including etching and crosshatching of small grooves into a cooling device and even into the die of an electronic device to be cooled, such technologies have required relatively elaborate steps in their design and manufacturing process. Commonly-assigned U.S. Pat. No. 7,219,715 to Popovich, the contents of which are incorporated herein by reference, describes an alternative approach using a mesh or woven screen that is between and bonded to two foils that define a flow cavity. A generic representation of this type of approach is depicted by a cooling device 10 shown in FIG. 1, in which a pair of foils 12 and 14 are bonded together, and a mesh 16 is contained within a cavity 18 defined by and between the foils 12 and 14. The interstices between the warp and weft strands 20 of the mesh 16, as well as the gaps between the strands 20 and the bordering foils 12 and 14, allow the passage of a cooling fluid, providing direct contact with the fluid for heat absorption and transfer heat through the bonding contacts with the foils 12 and 14. As further represented in FIG. 2, Popovich also provides an opening 22 in one of the foils 14 that provides for direct contact of the cooling fluid within the device 10 with the die 24 of an IC device, which is represented in FIG. 2 as projecting into the cavity 18 through the opening 22. FIGS. 3 through 4 represent variations of the cooling device 10 of FIG. 1. In FIG. 3, the device 10 is modified to include cooling fins 28 that promote heat dissipation to the surrounding environment. In FIG. 4, the device 10 is modified to include an integrated pump 30 for forcing the flow of the cooling fluid within the device 10. As would be expected, the fins 28 of FIG. 3 and the pump 30 of FIG. 4 can also be combined in the same cooling device 10.
An advantage of the cooling devices represented in FIGS. 1 through 4 is attributable to the tortuous course of the microchannels through the mesh 16, meaning that in addition to an X-Y labyrinth of interstices, a Z-plane of tortuousness is created that further increases the internal surface area for heat exchange between the fluid and the solid components of the cooling device 10.
Though the approach represented in FIGS. 1 through 4 is thermally efficient, exposure of the IC die 24 to the cooling fluid requires a sealing feature 32 (such as an adhesive seal, O-ring, etc.) around the opening 22 in the cavity 18 to allow direct fluid passage over the die 24. Aside from any potential reliability problems, the requirement for a sealing feature 32 can complicate the serviceability of the cooling device 10, and may render the device 10 ill-suited for aftermarket retrofitting by certain end users with limited technical skills.
In addition to Popovich, the use of microchannels for coolant fluids has been known for some time, as evidenced by U.S. Pat. No. 4,450,472 to Tuckerman et al. The preferred embodiment featured in this patent integrated microchannels into the die of the microchip to be cooled and coolant chambers. U.S. Pat. No. 5,801,442 also describes a similar approach. Still other approaches have focused on the combined use of coolant phase change (condensation) and microchannels, an example of which is U.S. Pat. No. 6,812,563. U.S. Pat. No. 6,934,154 describes a similar two-phase approach including an enhanced interface between an IC die and a heatspreader based on a flip-chip design and the use of a thermal interface material. U.S. Pat. Nos. 6,991,024, 6,942,018, and 6,785,134 describe electroosmotic pump mechanisms and vertical channels for increased heat transfer efficiencies. Variations of microchannel designs include vertical stacking of different orientational channel blocks as described in U.S. Pat. No. 6,675,875, flexible microchannel designs using patterned polyimide sheets as described in U.S. Pat. No. 6,904,966, and integrated heating/cooling pads for thermal regulation as described in U.S. Pat. No. 6,692,700.
Additional efforts have been directed to the manufacturing of microchannels. U.S. Pat. Nos. 7,000,684, 6,793,831, 6,672,502, and 6,989,134 are representative examples, and disclose forming microchannels by sawing, stamping, crosscutting, laser drilling, soft lithography, injection molding, electrodeposition, microetching, photoablation chemical micromachining, electrochemical micromachining, through-mask electrochemical micromachining, plasma etching, water jet, abrasive water jet, electrodischarge machining (EDM), pressing, folding, twisting, stretching, shrinking, deforming, and combinations thereof. All of these methods, however, share the drawback of requiring a more or less elaborate and expensive manufacturing process.